CIDR / Subnet Calculator

Parse a CIDR block (like 10.0.0.0/16) and instantly see the network address, broadcast, usable host range, subnet mask, wildcard mask, and total IP count. Useful for sizing VPC subnets, allocating IP ranges across teams, or sanity-checking a firewall rule. Runs entirely in your browser.

How to use the CIDR / Subnet Calculator

Paste a CIDR block in the form A.B.C.D/N where N is between 0 and 32. The tool returns: network address, broadcast address, usable host range (first to last assignable IP), subnet mask in dotted-decimal and binary, wildcard mask (the inverse), the total address count, and the number of /24, /25, /26 sub-blocks it contains. The shortcut buttons load common examples.

About CIDR notation

CIDR (Classless Inter-Domain Routing) notation describes an IP block in the form A.B.C.D/N. The /N says the first N bits identify the network; the remaining (32-N) bits identify hosts inside that network. /24 = 256 addresses; /16 = 65,536; /8 = 16.7 million.

The first address in a block is the network address (not assignable to hosts); the last is the broadcast address (also not assignable). So a /24 has 254 usable host IPs, not 256. The exceptions are /31 (point-to-point links per RFC 3021) and /32 (single host).

For VPC planning, /16 is a common default (~65K IPs per VPC, split into /24 subnets per AZ). Cloud providers (AWS, GCP, Azure) reserve a few IPs per subnet — typically 5 — so a /28 in AWS has 11 usable IPs, not 14.

Common use cases

• Sizing a VPC before creation — match address space to expected workload.
• Splitting a /16 into per-AZ /24s for high-availability deployment.
• Sanity-checking that two CIDR blocks don't overlap before peering.
• Translating a firewall rule between CIDR and wildcard mask notation (some legacy hardware uses wildcards).
• Quickly computing how many /28 subnets fit in a /24 (16).